1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries, such as lithium secondary batteries.
2. Description of Related Art
A non-aqueous electrolyte secondary battery that adopts lithium-containing nickel oxide or lithium-containing cobalt oxide as its positive electrode active material has a voltage of about 4 V and shows high capacity. For this reason, research and development on this type of battery have been actively conducted. A non-aqueous electrolyte secondary battery using gamma-butyrolactone (γ-butyrolactone) as a solvent for an electrolyte solution has been regarded as promising because gamma-butyrolactone has a high boiling point and is thermally stable.
However, a problem with a non-aqueous electrolyte secondary battery using gamma-butyrolactone as a solvent for the electrolyte solution has been that it cannot attain sufficient storage performance in a charged state in comparison with batteries using as a solvent a chain carbonate such as diethyl carbonate. A possible cause of the problem is decomposition of gamma-butyrolactone on the positive electrode or the negative electrode.
Japanese Unexamined Patent Publication No. 2003-7299 discloses that the use of a positive electrode active material having aluminum phosphate or the like on the surface improves charge-discharge capacity characteristics, cycle performance, thermal stability, and productivity in a battery that has an electrolyte solution using a mixed solvent of ethylene carbonate and dimethyl carbonate. However, the publication does not mention storage performance in the case of using gamma-butyrolactone. Japanese Unexamined Patent Publication No. 11-191417 discloses that the deterioration in characteristics of a battery exposed to high temperatures temporarily is suppressed by adding such a compound as aluminum phosphate to the positive electrode, the negative electrode, or the electrolyte solution to suppress the reaction between the active material and the electrolyte solution. According to the publication, however, the duration for which the battery is exposed to high temperatures is short, only several minutes, and the publication does not disclose any improvement in a long-term storage performance in a charged state. Moreover, the publication does not contain any disclosure regarding the storage performance in the case of using gamma-butyrolactone as a solvent.